There are a number of applications in which it is of interest to detect or image an object. Detecting an object determines the absence or presence of the object, while imaging results in a representation of the object. The object may be imaged or detected in daylight and/or in darkness, depending on the application.
Wavelength-dependent imaging is one technique for imaging or detecting an object, and typically involves detecting one or more particular wavelengths that reflect off, or transmit through, the object. In some applications, only solar or ambient illumination is required, while in other applications additional illumination is needed. Light is transmitted through the atmosphere at many different wavelengths, including visible and non-visible wavelengths. Thus, the wavelengths of interest may not be visible.
FIG. 1 is a diagram of the spectra of solar emission, a light-emitting diode, and a laser. As can be seen, the spectrum 100 of a laser is very narrow, while the spectrum 102 of a light-emitting diode (LED) is broader in comparison to the spectrum of the laser. And solar emission has a very broad spectrum 104 in comparison to both the LED and laser. The simultaneous presence of broad-spectrum solar radiation can make detecting light emitted from an eyesafe LED or laser and reflected off an object quite challenging during the day. Solar radiation can dominate the detection system and render the relatively weak scatter from the eyesafe light source small by comparison.
Additionally, the object being detected may not remain stationary during successive measurements. For example, if a human being is the object, the person may shift position or move during the time the measurements are taken. If measurements made at different wavelengths are made at different times, movement of the object during successive measurements can distort the measurements and render them useless.